This invention relates to opto-electronic scale reading apparatus of the kind comprising a scale defined by spaced lines, a read head, the read head and the scale being relatively movable in the direction of spacing of the lines; a plurality of units embodied in the read head and each comprising a light emitter for illuminating a region of the scale, a light receiver for receiving light from the illuminated region, grating means arranged in the light path between the emitters and the receivers for producing in respect of each unit a light modulation corresponding to the relative movement of the scale and the read head, wherein the light modulations of the respective units occur in phase-shifted relationship. The phase-shifted relationship is required for determining direction of movement and, possibly, position interpolation between scale lines.
Such an apparatus is known generally from GB No. 1,504,691 wherein the light modulations are in the form of moire fringes and wherein the units are positioned to read the fringes in phases separated by 90.degree.. The units are positioned to read the respective phases from different portions of the scale lines along the length thereof. As a result, if there is any yaw misalignment between the read head and the scale, the resulting moire error leads to a significant phase error with consequent difficulty regarding interpolation. Yaw misalignment is angular misalignment about an axis normal to the length and to the direction of spacing of the scale lines. A moire error is an error in the period of the moire fringes. A phase error is said to occur if the phase separation between said units differs from their nominal value of 90.degree..
Also, since the known apparatus reads the respective phases from different portions of the scale lines along the length thereof, the apparatus is sensitive to variations in such parameters as straightness, parallelity and reflectivity along the length of those lines.